This invention relates to a mass spectrometer sampling system and more particularly to a means for introducing small quantities of liquid from a continuous flowing stream into the high vacuum environment necessary in conjunction with mass spectroscopic analysis. This invention further relates to a means for thorough mixing of gas streams or vaporized liquid streams.
Mass spectrometers are devices used in pure and applied sciences as a means of identifying the existence of particular elements and compounds. The device enables such identification though the precise measurement of atomic masses. Gas or vapor to be characterized flows into a continuously pumped vacuum chamber at such a rate as to enable the chamber to be maintained at an extremely low pressure. Typical operation of such devices requires vacuums on the order of 10.sup.-8 to 10.sup.-6 torrs (mm of mercury). Molecules of the substance to be characterized are bombarded by an electron beam, typically emitted by a heated tungsten filament. Electrons of the beam collide with molecules of the substance to be characterized and strip off electrons, thus generating positively charged ions. These positively charged ions are accelerated across a series of charged plates. The ion beam is then directed to pass through a magnetic field which is oriented to deflect the beam. The extent to which the ion beam is deflected by the magnetic field depends on both the charhge and mass of the ion particles. The greater the charge of the ion, the greater its deflection. The deflection of an ion is further inversely related to its mass. In practice, the excitation of the electromagnet which provides the deflecting magnetic field is modulated and the ions are detected with either a collector plate or an electron multiplier.
Although various designs of mass spectrometer sampling systems for a liquid stream have been employed in the past, a continued problem faced by designers and users of such systems relates to the difficulty of reducing the pressure of the substance to be characterized from a relatively high pressure to the extremely low operating pressures of the system. In accordance with one experimental laboratory method, samples are periodically taken from a liquid stream by a piston-type pump designed to displace a very small volume of liquid. The outlet side of the pump is at a low pressure, typically about one torr. This low vacuum is reduced still further to that necessary for operation of the mass spectrometer through the use of a molecular leak, which provides an extremely high restriction to the flow of the vaporized liquid, thus enabling the necessary reduction in pressure. After a sample is taken, the system must be evacuated to permit the introcuction of a second sample. This process is laborious. More importantly, however, present day sampling pumps invariably possess a measurable degree of leakage due to the great pressure difference acting across the pump seals. Consequently, the purity of the sample is disturbed. Additionally, since a vacuum pump must be operated to provide the necessary low vacuum, there is a tendency for the samples to accumulate within the pumping system.
Another laboratory approach toward reducing the pressure of a substance to be characterized for mass spectroscopic study involves the use of a needle valve within a liquid stream which controls the flow of the liquid into a high vacuum chamber. Additional pressure reduction is provided through a molecular leak, such as described above. The disadvantages associated with this system includes those described above. Further, this system type suffers from a tendency for the needle valve to plug.
In addition to addressing the above-mentioned shortcomings of prior art sampling systems, there is a further need to provide a sampling device which enables the sampling of substances during a reaction process for real time analysis. It is further desirable for such a system to be mechanically simple as well as to be entirely closed such that toxic substances not being sampled into a mass spectrometer can be trapped or destroyed before venting.
The improved sampling system for a mass spectrometer device in accordance with this invention provides the above-mentioned desirable features. The system injects a liquid sample into an inert gas stream such as Helium at atmospheric pressure. Upon injection, the liquid is heated to cause it to vaporize and the vapor is carried by the inert gas stream into a plurality of isolated volumes connected in series. These volumes have the effect of diluting the concentration of the vaporized liquid injected into the inert gas stream and also shaping the concentration time profile at the exit of the last chamber to be a symmetric peak. At the conclusion of the sample flow path, a molecular leak or a membrane probe is provided which enables a small amount of the injected vaporized liquid to be introduced into the spectrometer ionized section. One significant advantage of the improved sampling system is that pump leakage is eliminated since the pump is not operating across a high pressure differential. Also, the pump does not have plugging problems since the piston moves into and out of the liquid flow stream such that impurities are flushed away. Additionally, a vacuum pump is not required which makes the system to be mechanically simple. Furthermore, the inert gas stream serves the purpose of continuously clean the walls of the sample volumes as well as the mass spectrometer ion source. The provision of the plurality of isolated volumes enables the concentration of the sample being characterized to be predicted accurately with respect to time.
When conducting mass spectrometric studies which involves the evaluation of a reaction employing gas or vapor reagents, problems have been presented due to poor mixing of the reagents. Inadequate mixing results in erratic reaction progression. In accordance with another aspect of this invention, a series of discrete volumes are employed within the inlet portion of a chemical reactor which thoroughly mixes the reagents prior to introduction into the reactor.
Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which this invention relates from the subsequent description of the preferred embodiments and the appended claims, taken in conjunction with the accompanying drawings.